1. Field of the Invention
This invention relates to pneumatic radial tire for passenger car, and more particularly to an improvement of a belt in a low-section profile radial tires running at an extremely high speed.
2. Disclosure of the Related Art
Recently, is possible to stably run a vehicle at an extremely high speed exceeding 150 km/hr with the technical innovation for passenger cars. Hence lowsection profile radial tires having an aspect ratio of not more than 0.6 (ratio of section height to maximum width in the tire) and sufficient performances even in the running at such an extremely high speed have been developed.
However, when the tire is run at extremely high speed, separation between tread and belt, chipping of tread rubber and the like particularly become a problem, and consequently it is important that the durability to these failures is high or a so-called high-speed durability is excellent.
Considering high-speed durability, there is proposed a tire structure that a rubberized fabric of organic fiber cords is spirally wound around an outermost belt layer arranged outside a carcass under a constant tension.
In general, such a tire develops sufficient performance in case of running at middle to high speeds, but is hardly said to possess sufficient performance in case of running at an extremely high speed. That is, it is required to solve the following problems.
In this tire, the tread is provided at its surface with plural circumferential grooves of relatively wide width along the circumference of the tread considering the drainage property and has a thickness that it is thin at a position of the circumferential groove and becomes thick at a position of block row between the adjoining circumferential grooves. The difference of rubber volume between the circumferential groove and the block row becomes large and consequently the protruding quantity of rubber in an outward direction of the tire through centrifugal force produced in the extremely high-speed running becomes largely different in the widthwise direction of the tread. That is, the protruding quantity at the position of the circumferential groove is slight, but the protruding quantity in the block row is large, so that the ground contact pressure of the tread becomes unequal between the position of the groove and the block to injury the steering stability in the extremely high-speed running and cause the occurrence of uneven wear.
When extremely high-speed running is continued, heat is generated due to the excessive ground contact pressure of the block row through the centrifugal force, which exceeds the heat-resistant limit of the tread rubber to cause blowing-out thereof and hence the peeling of rubber in the block row or the occurrence of so-called chunk-out. In this case the tire life becomes very short.